1. Field of the Invention
The present invention relates to an inkjet printhead substrate, inkjet printhead, and inkjet printing apparatus.
2. Description of the Related Art
In a general thermal inkjet printhead, discharge heaters for discharging ink, and lines for electrical connection are formed on the same substrate, and nozzles for discharging ink are formed on them.
Recently, inkjet printing techniques are actively expanding to large-format printing presses, commercial printing presses, and like. Large-format printing presses, commercial printing presses, and like are larger in printing amount and higher in print processing frequency, compared to home printers, office copying machines, and the like. In other words, to promote expansion of inkjet techniques to large-format printing presses, commercial printing presses, and the like, inkjet printheads need to be more durable than conventional ones.
One means for improving the durability of the inkjet printhead is a discharge failure detection & discharge failure compensation technique. The discharge failure detection & discharge failure compensation technique is a technique of minimizing adverse effects on a printed image even if a predetermined number of heaters become unavailable. In general, if even one heater fails, a corresponding nozzle does not discharge an ink droplet, generating a defect such as a white stripe in a printed image. It is determined that the inkjet printhead fails in the printing operation when discharge failures occur in a relatively small number of heaters. However, the discharge failure detection & discharge failure compensation technique can be used to suppress degradation of a printed image caused by a predetermined number of discharge failures. This technique allows increasing the heater trouble count threshold at which it is determined that the printhead fails in the printing operation, improving the durability of the printhead.
A known example of the discharge failure detection technique is a technique disclosed in Japanese Patent Laid-Open No. 07-032608. In this technique, a failed heater is specified using a vibration plate. An ink droplet is discharged to the vibration plate capable of electrically or magnetically detecting a displacement. A failed nozzle is detected based on the presence/absence of a displacement of the vibration plate.
A known example of the discharge failure compensation technique is a technique disclosed in Japanese Patent Laid-Open No. 2006-231857. In this technique, the interval between the driving timings of heaters adjacent to each other is suppressed to be equal to or shorter than the cycle of ink refill to the nozzle. At one discharge timing, a nozzle adjacent to a discharge failure nozzle discharges ink droplets twice. Generally in the discharge failure compensation technique, another heater corresponding to a discharge failure heater is used as an alternative heater.
In addition to the durability of the printhead, downsizing is also requested of the printhead substrate. One factor which defines the substrate size is securement of the heater line region. For example, sharing heater lines is very effective for reduction of the substrate size. When sharing heater lines, it is necessary to minimize the influence of the heater line resistance difference in the heater array on the ink droplet discharge performance.
FIG. 10 is a view exemplifying the arrangement of an inkjet printhead substrate in which heater lines are partially shared. Heater arrays 210 are respectively arranged on the two sides of an ink supply port 240. Power supply pads 231 are prepared at the two ends of one heater array. Further, the heater array 210 is divided into six blocks, and heaters belonging to the respective blocks are connected to a common heater line 220. The line width of the common heater line 220 is adjusted in accordance with the distance to the power supply pad 231 so that the heater line resistances of the respective blocks become almost equal to each other.
The enlarged view of a region K showing some of the blocks of the heater array 210 shows heaters 211 and switching elements 251. The enlarged view also shows a driving voltage-side common heater line 221 connected to a heater driving voltage supply pad, a ground-side common heater line 223 connected to a ground voltage supply pad, a driving voltage-side individual heater line 222, and a ground-side individual heater line 224. The switching element 251 is formed using a lower conductive layer and gate electrode layer. The switching element 251 is arranged in correspondence with the heater 211 and heater line via a heat storage layer (second heat storage layer 150). The switching element 251 is electrically connected to the ground-side individual heater line 224 via a through hole 264 serving as the opening of the second heat storage layer 150, and further electrically connected to the ground-side common heater line 223 via a through hole 263. In the conventional technique typified by FIG. 10, the driving voltage-side common heater line 221 is laid out by folding it back at the heater block end while the ground-side common heater line 223 is laid out straight. This layout makes line resistances in the heater blocks to be almost equal.
Further, the heater 211 and heater line are covered with a passivation film layer made of an insulating material, and are protected from ink or the like. At a portion of the passivation film layer that corresponds to the heater 211, an anti-cavitation layer is formed to protect the heater 211 from cavitation generated when ink is bubbled and discharged.
FIG. 11 is a view exemplifying the arrangement of an inkjet printhead substrate in which heater lines are completely shared. The heater arrays 210 are respectively arranged on the two sides of the ink supply port 240. The power supply pads 231 are arranged at the two ends of one heater array. All heaters belonging to the heater array are connected to the common heater line 220. This heater line arrangement is effective when the resistance component of the common heater line is much lower than that of the heater line, and the influence of the heater line resistance difference between the end and center of the heater array on the ink droplet discharge performance is small. That is, as long as the influence on the ink droplet discharge performance and the like are small, the arrangement shown in FIG. 11 can implement a more efficient line layout than the partially shared heater line as shown in FIG. 10.
As described above, sharing heater lines is effective for realizing an efficient line layout. However, in the use of this arrangement, discharge failures readily occur in chains in successive heaters.
A mechanism of successively generating discharge failures in a plurality of heaters will be explained. FIG. 12A exemplifies the periphery of heaters on a printhead substrate in which heater lines are partially shared. FIG. 12A shows a state immediately after a passivation film layer covering one heater is disconnected due to some influence, and a heat resistance layer which forms the heater, and an individual heater line contact ink. A heater which fails in discharge is shown in a frame of a broken line.
If the passivation film layer is disconnected to directly expose the individual heater lines 222 and 224 to ink, the heater line material elutes over time, and corrosion of the heater line proceeds. Note that L is corrosion of the heater line. In this specification, corrosion means a line state in which a line is ionized and dissolved by ink to generate a heater discharge failure.
FIG. 12B is a view exemplifying a case in which the printhead substrate shown in FIG. 12A is kept used after the passivation film layer is disconnected. In this case, the corrosion L of the heater line propagates to the driving voltage-side common heater line 221, cutting off supply of the heater driving voltage to heaters 213 connected to the same common heater line. As a result, even heaters connected to the same common heater line as that of a heater in which a discharge failure has occurred first become unavailable.
In this case, the above-mentioned discharge failure detection & discharge failure compensation technique may be applied. However, the discharge failure compensation technique uses another heater as an alternative heater to compensate for a heater suffering a discharge failure. If the number of heaters having discharge failures increases, discharge failure compensation is impossible. In compensation using adjacent nozzles, if heater troubles concentrate at a specific portion, color irregularity cannot be satisfactorily reduced.
That is, to effectively improve the head durability by discharge failure detection and discharge failure compensation in a printhead having a printhead substrate in which heater lines are shared, an arrangement which suppresses the phenomenon in which discharge failures are successively generated in a plurality of heaters is required.
To solve the above problem, Japanese Patent Laid-Open No. 2006-51770 proposes a technique of arranging an electrode using a corrosion-resistant metal in the boundary region between the heater line portion and the heater portion, and suppressing propagation of corrosion of a line to another heater even if a passivation film layer corresponding to one heater is disconnected. However, this technique requires an additional corrosion-resistant metal layer step in the substrate formation process. This may raise the substrate cost and decrease the productivity.